A new way to generate extreme-UV emissions using intense laser pulses, discovered by a team at the Institute for Basic Science (IBS), promises to find applications in high-resolution imaging, lithography, and ultrafast spectroscopy.

When a strong laser shines on helium gas atoms, electrons transition from ground to excited state. The excited atoms then emit light corresponding to the energy difference between the two states and the electrons come back to their original ground state. The general belief is that this happens when the atoms absorb several light particles. However, according to the IBS research, the journey of the electrons can take a different path: When the intensity of the laser field is high, the electrons can experience frustrated tunneling ionization (FTI). Rather than coming back straight away to the ground state, they can remain floating near the atom, in the so-called Rydberg states. In this case, the emitted light depends on the energy difference between Rydberg and ground states. Courtesy of IBS. The IBS team produced coherent extreme-UV emission via frustrated tunneling ionization (FTI). The researchers found that the intensity of the emission depended on the ellipticity and carrier-envelope phase of the laser pulses. They also discovered that the propagation direction of the emission could be coherently controlled using the attosecond lighthouse technique.

The figure on the top compares FTI (blue dotted line) with high harmonic generation (purple dashed lines). When a laser field (red solid line) is applied to a helium atom, an electron becomes free. Depending on the timing, the free electron accelerates with different trajectories. In the well-known case of high harmonic generation, the electron recombines with its parent atom, and the periodic repetition of this process generates bursts of light emission having the spectrum at regular intervals, as shown in the lower image. In the case of FTI, once the effect of the laser is gone, the electrons reach a Rydberg state. FTI-generated extreme-ultraviolet radiation shows a fairly large divergence angle and narrow spectral width. Courtesy of IBS. The researchers controlled the trajectory of electrons by manipulating characteristics of the laser pulse. In FTI, the electrons travel a much longer trajectory than in high harmonic generation, and thus are more sensitive to variations of the laser pulse. This allowed the team, for example, to control the direction of the emitted radiation by manipulating the wavefront rotation of the laser beam, using spatially chirped laser pulses.

“We used IBS’ state-of-the-art laser technology to control the movement of the meandering electrons,” said professor Kyung Taec Kim.

When a strong laser light is shined on atoms, their electrons are free to temporarily escape from the parent atoms. As the laser is turned off, these “meandering electrons” can either recombine with their parent atoms or continue to “float” nearby. Whether high-harmonic generation (the fast return of electrons to their parent atoms) or “floating” (FTI) occurs, the result is the emission of light with a specific wavelength.

Lasers with ultrafast pulses have provided the ability to monitor and control electrons with subatomic resolution, allowing scientists like the IBS researchers to better understand real-time electron dynamics and generate customized emissions.

“We could identify a completely new coherent extreme-ultraviolet emission that was generated,” Kim said. “We understood the fundamental mechanism of the emission, but there are still many things to investigate, such as phase matching and divergence control issues.

“These issues should be solved to develop a strong extreme-ultraviolet light source,” he said. “Also, it is an interesting scientific issue to see whether the emission is generated from molecules, as it could provide information on the molecular structure and dynamics.”

The research was published in Nature Photonics (https://doi.org/10.1038/s41566-018-0255-8).

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